1. Field of the Invention
The invention relates to a shift control apparatus and shift control method for an automatic transmission, which executes a torque reduction control when downshifting in response to a second downshift determination made during a first downshift.
2. Description of Related Art
Japanese Patent Publication Laid-Open No. 10-281277 describes a shift control for an automatic transmission that executes a shifting of a rotation transferred from an engine to an input member so as to be output to drive wheels. During a first downshift, in which a first friction coupling device is engaged, the operation of the accelerator by the driver may cause a second downshift command to be output. When the transmission executes the second downshift, the first friction-coupling device is disengaged and a second friction-coupling device is engaged. Accordingly, the engine may overspeed, and the second downshift cannot be performed appropriately. The described shift control addresses this problem by delaying the downshift, in spite of the downshift command, for a predetermined time. In addition, Japanese Patent Publication Laid-Open No. 8-244499 describes the technology for starting a downshift, which disengages the first friction-coupling device and engages the second friction-coupling device, when the downshift determination is made during an initial shift that engaged the first friction-coupling device, through the accelerator pedal operation. Then, the torque reduction control for reducing the torque of the engine is executed during the downshift to prevent the overspeeding of the engine and reduce shift shock.
In Japanese Patent Publication Laid-Open No. 10-281277, because the second downshift is delayed, there is a lag between the operation of the accelerator pedal and output of the desired driving force through the second downshift operation. Meanwhile, in Japanese Patent Publication Laid-Open No. 8-244499, the downshift operation starts immediately in response to the downshift command and the torque reduction control is executed simultaneously. The downshift operation is then performed under the condition where the torque of the power source is suppressed. Accordingly, much time is taken for the rotational speed of the input member to increase to the synchronous rotational speed of a gear stage after the downshift operation. This also results in a time lag between operation of the accelerator pedal operation and output of desired driving force.